Since 2001, the Department of Defense has spent more than $9 trillion on everything from new weapons systems to soldier salaries, and nearly 20 percent of that money — $1.7 trillion in total — has come from an “off-the-books” budget account called the Overseas Contingency Operations fund. It’s the best-known budget gimmick in Washington, a classic example of Democrats and Republicans finding common ground when they want to boost defense spending while technically abiding by the current budget caps.

But starting in 2017, a fierce critic of that gimmick is set to become one of the most important people in the Washington budget world. President-elect Donald Trump recently picked Rep. Mick Mulvaney to head the Office of Management and Budget, the small yet powerful agency that produces the president’s annual budget. It’s an extraordinarily powerful role, experts say, enabling Mulvaney to leave his fingerprints on every corner of the federal government — and, importantly, giving him a direct line to Trump on matters of federal spending.

Mulvaney has been one of the loudest critics of the fund during his time in Congress, hammering it as a “slush fund” and sponsoring legislation to eliminate it. Now, suddenly, he will find himself in a unique position to kill it off. Could he do it?

Experts say any serious effort to end the gimmick could set up a huge internal fight between the White House and the Pentagon. And even if he succeeds there, Mulvaney would then face an even tougher challenge of convincing his former colleagues in Congress to end the gimmick, forcing them to make difficult budgetary decisions in an era of tight resources. As longtime budget watchers know, when given the choice between making those tough choices and avoiding them through a gimmick, Congress typically chooses the latter.

On paper, the budget gimmick serves a real purpose. The Overseas Contingency Operations fund, or OCO, was established in 1997 and is typically referred to as an emergency fund; Congress appropriates money into the OCO accountfor the Pentagon to fund unplanned needs, like a war. During the George W. Bush administration, OCO defense spending increased dramatically as the U.S. tapped the account to pay for its wars in Afghanistan and Iraq. It peaked in fiscal 2008, at $187 billion, accounting for 28 percent of total Pentagon spending.

Since then, it has slowly declined as Obama has adopted a lighter-touch military strategy, pulling out troops in Iraq and Afghanistan. OCO defense spending in fiscal 2016 was $59 billion, 10 percent of total Pentagon spending.

Mulvaney calls this a “slush fund,” and few budget experts disagree. Its real role, they say, is to allow the Pentagon to keep spending money on overseas wars without violating the spending caps imposed by the 2011 budget deal. Todd Harrison, an expert on the defense budget at the Center for Strategic and International Studies, said that starting in 2014, the Pentagon began shifting its base defense spending into the OCO account, which — unlike the main budget — doesn't have a limit. That gave defense hawks in Congress and the Pentagon an easy way to boost defense spending without looking like they were violating the budget cap: Simply put the money in the OCO, and pretend that the defense spending caps aren’t violated.

In 2016, Harrison estimates, the department shifted about $30 billion from its base account into the OCO fund, a figure that is disputed by DOD officials. (The recently passed continuing resolution that funds the government until April 28 largely extended defense and OCO spending at their current levels.)

Mulvaney has been an avowed critic of such chicanery, offering numerous amendments to eliminate OCO spending and restricting its intended uses. (Most failed by wide margins.) In his new perch as budget director, he will have levers of power with which to go after OCO, but he’ll face new pressures as well.

For new presidents, the release of their first budget is a marquee event, laying out a detailed plan of the issues the new administration intends to prioritize in the upcoming year. With Republicans in control of both the House and Senate, Trump’s first budget, due by February 6, will carry even more weight. If Mulvaney really wants to prioritize ending the OCO budget gimmick, he could simply direct the Department of Defense to create a budget without OCO funding. That would be a tough task: If the DOD adhered to the caps on defense spending, it would force the department to make huge spending cuts.

“It would force an issue within the administration and quite frankly, it would enforce a much higher degree of fiscal accountability,” said Harrison.

Such a directive from OMB would undoubtedly draw a sharp rebuke from top Pentagon officials, who could appeal to Trump to overrule Mulvaney. It’s unclear how the president-elect would ultimately come down. He has spoken frequently about increasing the size of the military, and has chosen generals, not civilians, for many of his top security advisory posts, including the secretary of defense.But he has also, at times, singled it out for cuts and has recently become personally involved in the contract negotiations for major Pentagon procurement issues, including over the costly F-35 program.

Obama and his current OMB director, Shaun Donovan, have frequently argued for an end to the OCO fund so that the Department of Defense is forced to account for all of its spending in its real budget. But the White House’s budgets haven't backed that up, continuing to use OCO as a way around spending caps. At issue for the Obama administration is the president’s position that every dollar increase in defense spending be accompanied by a dollar increase in non-defense, a demand strongly opposed by congressional Republicans.

With Trump taking office, Republicans will be in a stronger position to boost defense spending next year, potentially by taking money away from non-defense spending, like human services and national parks — a significant fear on the left. Defense hawks are already reportedly insisting that Congress lift the budget cap on defense spending specifically, giving the Trump administration a potential opportunity to end OCO spending while not forcing large cuts to the Pentagon.

Still, it’s tough to predict what exactly will happen. Ending OCO is the exact type of challenge that meshes with Mulvaney’s fiscal hawkishness. Even liberals who vehemently disagree with Mulvaney say he has been consistent in his efforts to reduce the deficit. And to conservatives, he’s the perfect person for the job, someone with a track record of demanding tough cuts from the bureaucracy and a commitment to eliminate notorious budget gimmicks.

“Whether it’s in OCO or anything else, [Mulvaney says] let’s not bury the true costs,” said Rep. Bill Huizenga (R-Mich.), a close friend of Mulvaney. “Let’s be up front and intellectually honest and acknowledge what things are actually costing us as taxpayers.”

Whether he can finally take down Washington’s favorite budget gimmick is yet to be seen.